The present invention relates to digital communication systems and more particularly to systems and methods for transferring information related to control of the physical layer.
A point-to-multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose.
In order to conserve scarce spectrum, the data communication devices of a point-to-multipoint wireless communication system may share access to a common frequency. In a typical scenario, one or more frequency channels are allocated to downstream broadcast communication from a central access point to a plurality of subscriber units. One or more separate frequency channels are allocated to upstream communications from the subscriber units to the central access point. For upstream communication, a medium access control (MAC) protocol determines which subscriber unit is permitted to transmit at which time so as not to interfere with transmissions from other subscriber units.
For a given upstream frequency, the time domain is divided into frames which are typically of equal duration. Each frame represents an individually allocable unit in the time domain. One subscriber unit transmits in each frame. Reservations for transmission in a particular frame are made by the central access point and distributed in broadcast downstream transmissions.
It is useful to model the design of a network as consisting of multiple layers. Layers are arrange in a hierarchical fashion with each layer being built on top of a layer below it. The lowest layer is known as the physical layer and controls interaction with the physical medium. Each layer performs functions required by the layer above it and shields the layer above it from the details of implementation. A hardware or software entity implementing a given layer at a particular node of a network interacts with other entities of the same layer operating at other nodes in the network. Except for the physical layer, they do not interact directly but rather via the lower level layers at their respective nodes. In a point-to-multipoint communication system, various entities that implement the MAC protocol are collectively referred to as the MAC layer. The MAC layer is directly above the physical layer.
In many point-to-multipoint communication systems including, e.g., a data over cable system, the MAC layer is responsible for other functions besides coordinating the timing of upstream transmission so that subscribers do not interfere with one another. For example, the MAC layer may be responsible for regulating the upstream power transmission of individual subscriber units to prevent saturation of the central access point receiver and to minimize interference to unintended receivers. In order to coordinate upstream transmission in the time domain, it may also be necessary to establish the propagation delay between the central access point and individual subscriber units. The process of establishing these propagation delays is known as ranging and may be also a function of the MAC layer.
The discussion so far has concerned point-to-multipoint wireless communication systems. Cable modem systems also involve access to a shared medium, the cable infrastructure. It would be desirable to simply adopt a MAC protocol already developed for cable applications to the wireless context. One such protocol that has been developed is referred to as the MCNS protocol. The MCNS protocol is described in Data-Over-Cable Service Interface Specifications, Radio Frequency Interface Specification, SP-RFI-I04-980724, (Cable Television Laboratories, 1997), the contents which are herein incorporated by reference.
A cable MAC layer like MCNS is a already implemented in low cost chip sets. The operational characteristics of MCNS are well known. Higher layer protocol hardware and software has been developed to interact with MCNS. Furthermore, it is desirable to maintain parts commonality between wireless modems and cable modems to the extent possible. Unfortunately, many features of wireline MAC protocols are not appropriate for the wireless context. For example, a wireless communication system may require much more frequent updates of subscriber unit power level than does a wireline system to accommodate changes in propagation conditions. This creates an excessive processing burden on the wireline MAC processor. Also, subscriber units periodically transmit to the central access point even when they have no new information to transmit solely for the purpose of providing a power measurement level update. Each such transmission by a subscriber unit requires an entire MAC frame. Frequent updates of power control information necessitated by the wireless channel consume precious bandwidth.
One solution to this problem of providing adequately frequent upstream power measurements in a system employing a wireline MAC protocol is described in the patent application entitled WIRELESS POWER CONTROL IN CONJUNCTION WITH A WIRELINE MAC PROTOCOL. In the system described there, upstream MAC frames are further subdivided into subframes that are controlled by the physical layer rather than by the MAC layer. Some of the subframes are allocated to upstream power measurements. However, creating such a physical layer frame structure requires downstream transmission of the physical layer control information. For example, there must be scheduling information transmitted to the subscriber units to define access to the physical layer frames.
Prior art implementations of point-to-multipoint communication system downstream physical layers have focused on simply transferring information generated by the MAC layer from the central access MAC layer entity to one or more subscriber unit MAC layer entities. What is needed is a system that transfers physical layer control information from the central access point to individual subscriber units with complete transparency to the operation of the MAC layer.